1. Field of the Invention
The present invention relates to an imaging apparatus having an imaging system to which a hemispherical dome cover can be attached.
2. Description of the Related Art
Many surveillance cameras have an imaging system enclosed in a hemispherical dome cover attached thereto. The use of a dome cover in a surveillance camera protects the imaging system and prevents unauthorized users from determining the imaging direction of the camera. The dome cover, however, has a concentric shape and a thickness of several millimeters which may deteriorate the imaging capacity of the imaging system. In other words, the dome cover itself has a lens action (optical action), and therefore attaching the dome cover to an imaging apparatus changes the optical characteristics of such apparatus. More specifically, attaching a dome cover having a lens effect to an imaging system significantly changes the focal length and the focus position of the entire system. The change in the focal length of the entire system incidentally changes the photographing angle of view of the imaging system. For this reason, in an imaging apparatus having an imaging system to which a dome cover can be attached, it is important to reduce not only the change in focus but also the change in optical performance including the change in focal length caused by attaching the dome cover. The amount of change in focal length (defocus) caused by attaching the dome cover varies significantly depending on the relationship between the focal length of the dome cover itself and the focal length of the imaging system. Defocusing issues of type described above have been previously considered, and imaging apparatuses are known in which such defocus is corrected.
A lens apparatus described in U.S. patent application publication US 2007/0292121 prevents defocus by performing focus adjustment of an imaging system with a member attached that has the same optical characteristics as a case member, and removing the member and attaching the case member. A monitoring camera described in Japanese Patent Application Laid-Open No. 2008-172423 controls the amount of defocus of a focus lens on the basis of information on a case member and information on an imaging optical system. An imaging apparatus disclosed by U.S. patent application publication US 2005/0285972 has a cylindrical window section in front of a lens and corrects the difference in focal length due to the difference in refractive power between the vertical direction and the horizontal direction of the window section. More specifically, in US 2005/0285972, a correction plate cylindrically bent in the direction perpendicular to the bending direction of the window section and having the same bending radius as the window section is disposed between the window section and the lens. In other words, a sampling of the known imaging apparatuses having a dome-shaped cover reveals that these apparatus namely address defocusing conditions.
However, in the case where the imaging system has a zooming function like a zoom lens or a varifocal lens, the amount of change in focal length caused by attaching the dome cover differs significantly between the wide-angle end and the telephoto end. Therefore, the zoom ratio of the entire system including the dome cover is significantly changed by attaching the dome cover, and the optical specification is significantly changed. In addition, if the defocus caused by attaching the dome cover is adjusted by moving a focus lens group, the range of a focusable object distance is changed. Further, aberrations (spherical aberration, coma aberration, astigmatism, field curvature, distortion, chromatic aberration of magnification, axial chromatic aberration) of the imaging system are also changed by attaching the dome cover. Accordingly, there is a need in the state of the art for dome-covered imaging apparatuses that can adequately address changes in zooming and minimize aberrations.